Inductive power supply systems allow power to be transferred to an electronic device, such as a portable device, without the need for direct electrical connections. Inductive power transfer may be achieved using inductors, which produce magnetic fields when current flows through them. Conversely, current may be induced in an inductor when in the presence of a magnetic field, such as the magnetic field produced by another inductor. If two inductors are placed in proximity and one inductor is driven with a current, then the other inductor will produce a current even though the two inductors are not directly connected. This interrelationship between the two inductors is generally called inductive coupling, and many have used this phenomenon to transfer power without electrical connections.
In fact, many of the fundamental principles of wireless power transfer have been known for 100 years or more. Nicola Tesla, who is widely regarded as the father of wireless power transfer, is reputed to have demonstrated a system for wirelessly powering a light bulb as early as 1893. Tesla spent many years conducting research and development in the field, and amassed a significant portfolio of patents relating to wireless power transfer. As we see a resurgence of interest in wireless power, some of his early inventions are being used by those developing wireless power systems today. For example, U.S. Pat. Nos. 649,621 and 685,012 to Tesla disclose that inductive power transfer between a primary coil and a secondary coil may be improved by incorporating an additional set of intermediate coils that function as “resonating” coils to magnify the oscillations and communicate power between a primary unit and a secondary unit. More specifically, the primary unit includes a pair of coils that work together to transmit power to the secondary unit and the secondary unit includes a pair of coils that work together to receive the power. The primary unit includes a primary coil that is electrically connected to and directly receives power from the power source, as well as a resonating coil that is coupled inductively to the directly-powered coil. The resonating coil receives power inductively from the primary coil, magnifies the oscillations, and generates an electromagnetic field to communicate the power to the secondary unit. Tesla also demonstrated that capacitance used in combination with the resonating coil may produce even larger oscillations than the resonating coil by itself. The secondary unit includes another resonating coil that receives the electromagnetic field generated by the primary unit resonating coil and a secondary coil that is inductively coupled to the secondary resonating coil to directly transmit power to the secondary load. So, as can be seen, the concept of using a separate set of intermediate coils to provide an inductive coupling with improved performance has been known for over a century.
Although the basic concepts of wireless power transfer have been around for many years, there has been a relatively recent resurgence in the interest in the technology, and widespread efforts are being made to implement practical and efficient wireless power transfer systems. There are a variety of factors that complicate development of efficient systems. For example, operating characteristics (i.e. conditions under which the system is operating) can have a significant impact of the quality and efficiency of the power transfer. As another example, mutual inductance can have a material impact on the efficiency of the power transfer between the primary unit and the secondary unit. Mutual inductance depends on a number of circuit parameters, including the distance between the primary unit resonating coil and the secondary unit resonating coil. As the distance between the primary unit resonating coil and the secondary unit resonating coil is minimized, the mutual inductance increases. This inverse relationship between the distance and the mutual inductance may impose restrictions on the operating parameters of the system.
The energy transfer efficiency of the primary unit and secondary unit may be improved by varying the operating parameters of the power supply system to accommodate different operating conditions. As a result, high efficiency power supply systems have been developed that adapt the operating parameters of the power supply in response to changing operating characteristics, such as relative distance and orientation between the primary unit and the secondary unit (or receiver unit). Adaptive power supply systems may vary operating parameters, such as the resonant frequency of the primary unit or the secondary unit, or the operating frequency, duty cycle or rail voltage of the drive signal. However, variable drive frequency solutions tend to rely heavily on closer proximity or higher mutual inductance to control power transfer efficiency between the primary unit and the secondary unit. As a result, variable drive frequency solutions generally lack spatial freedom between the primary unit and the secondary unit.
In variable resonant frequency systems, the spatial freedom may be greater than a variable drive frequency system because the system is not as reliant on close proximity or higher mutual inductance. However, precise control over power transfer efficiency may be more problematic because variable resonant frequency systems are not as finely tunable as the variable drive frequency solutions. This is why a variable resonant frequency system may produce mixed results during actual use.